Recently, there has been a trend for photodisk and magnetic disk devices to decrease in size and weight and increase in capacity. Popular use of notebook-sized personal computers has made it imperative to prepare spindle motors of small size and decreased thickness, and as the development of operating systems and applications advances, HDD devices (hard disk drivers) are required to have an increased capacity. Further, by decreasing the flight height of the head or by using an MR head, an increase in the capacity of small-sized thin HDD devices may be attained. To this end, it is necessary to minimize the warping;
This kind of spindle motor has a rotor frame attached to a hub formed with a support surface for the disk, said hub portion being driven for rotation by magnetic repulsive action produced between the housing body and the rotor frame.
If the processing accuracy of the top surface of said rotor frame is poor, or if there is a difference in thermal expansion coefficient due to a difference in material between the rotor frame and the hub portion, or if the contact surfaces of the rotor frame and hub portion are adhesively bonded, a deformation takes place in which the outer periphery of the flange portion which serves as the support surface of the hub portion for the disk rises above the level of the inner periphery, and a warp develops in the disk attached to the flange portion, adversely affecting the reading and writing of signals from and in the disk.